This invention relates to a heat exchanger and a method of manufacturing the heat exchanger which is a high thermal efficiency and a high boring corrosion resistance, and more particularly to a heat exchanger manufactured by brazing aluminum or aluminum alloy heat exchanger members to each other with a corrosion-resistant flux.
Heat exchangers are used in, for example, domestic air conditioners, refrigerators, and automobile air conditioners, and utilize a fluid flowing as a cooling medium or a heat medium through a metal passage provided with a large fluid-contact area. When the fluid-contact area of a metal passage is increased, the shape of the metal material forming the passage inevitably becomes complicated. In a heat exchanger consisting of tubes and fins which are provided to increase the fluid-contact area of the fluid passages, the tube are set in holes and grooves formed in the fins. The tubes are then expanded by mechanical force to attach the tubes and fins to each other. The tubes fitted firmly into the fins are connected together by attaching U-shaped tubes, which have been molded in advance, to end portions of the tubes. Flux consisting of a chloride is used for brazing the joints between the tubes fitted into the fins and the U-shaped tubes.
A heat exchanger consisting of fins and tubes is manufactured in this manner, i.e. in two connection steps. In the first step, the fins and tubes are joined together mechanically, and in the second step these tubes and U-shaped tube are brazed together. When viewed microscopically, the mechanical connections between the fins and tubes is a contact between the oxide films formed on the materials thereof. This reduces the rate of transfer of heat between the fins and tubes.
Gaps tend to occur between the mechanically-connected portion of the fins and tubes because of vibrations generated while the heat exchanger is being transported and the heat cycle it is subjected to after it has been manufactured. This also results in a large reduction in the heat transfer rate of the fins and tubes. During the manufacture of a corrugated heat exchanger which must have a high corrosion-resistance, flux to which ZnCl.sub.2 is added is used. Since a corrugated heat exchanger has fins of complicated shape, it must be subjected to many steps, such as a cleaning step and an acid-washing step after the brazing step. Moreover, the residual Zncl.sub.2 cannot be removed completely.
A method of brazing aluminum using a corrosion-resistant flux of a fluoride which is used to eliminate these problems, i.e. a flux consisting of a compound of K.sub.3 AlF.sub.6 and KAlF.sub.4 and which is obtained by melting and mixing together KF and AlF.sub.3, is proposed by U.S. Pat. No. 3,951,328. This corrosion-resistant fluoride flux has a low hygroscopicity, and, therefore, it is unnecessary to remove the flux remaining on the joint surfaces of the heat exchanger after the brazing step. Although the residual flux has no corrosive components, there is still room for improvement in the boring corrosion resistance of such a heat exchanger.